IGFETs of the floating-gate type are well known in the art; see, for example, U.S. Pat. No. 3,825,946. They comprise two main electrodes, known as a "source" and a "drain", contacting respective enclaves of one conductivity type (usually n) located in a silicon substrate of the opposite conductivity type (p) at opposite ends of a channel of their own conductivity type whose resistivity depends on the electric charge of the floating gate. The latter generally consists of an insert of polycrystalline silicon embedded in a layer of silicon oxide overlying the substrate. Two accessible gates closely spaced from the inaccessible floating gate, i.e. a writing gate and a cancellation gate, are respectively coupled therewith via a relatively high and a relatively low capacitance. To charge the floating gate, the two accessible gates and the drain electrode are driven highly positive with reference to the source electrode and the substrate whereby electrons pass at high velocity through the wide-open channel and are partly attracted by way of the intervening oxide-layer portion into the floating gate. This charge increases the resistivity of the channel so that a voltage difference higher than before will be required between the two accessible gates and the source electrode in order to render the IGFET conductive.
To discharge the floating gate, the cancellation gate is driven highly positive with reference to the writing gate and the source and drain electrodes whereby a significant portion of the electrons previously accumulated in the floating gate are extracted through the portion of the oxide layer separating the cancellation gate from the floating gate. It has been found, however, that the time or the gate-biasing potential required for cancellation--i.e. for lowering the conduction threshold of the IGFET to a predetermined level--progressively increases with the number of reprogramming operations, presumably on account of a reduced conductivity of the oxide layer due to the trapping of electrons therein. This phenomenon limits the number of times a given cell can be reprogrammed before excessive time or voltage requirements render it practically unusable.
The described aging process, due not so much to length of service as to the number of writing and cancellation operations, differs for the various cells of the memory in accordance with their individual history. Thus, a cell in a virgin or near-virgin state can be discharged relatively quickly as compared with one that has undergone a large number of charge modifications. To a much lesser extent, this applies also to the time required for the writing of a cell, i.e. for the raising of the conduction threshold of its IGFET to a predetermined elevated level.
If, during reprogramming, the cancellation gate of any cell is subjected to high voltage for an excessive period, the electron population of its floating gate may be depleted to a point known as overcancellation which unduly lengthens the time required for recharging same in a writing operation. Conversely, overcharging the floating gate during writing will further lengthen the time required for subsequent cancellation and will also tend to accelerate the deterioration of the oxide layer. For these reasons, the indiscriminate charging or discharging of a plurality of cells with different histories upon reprogramming inevitably leads to an unsatisfactory performance of some of these cells and/or to a rapid obsolescence of the memory.